Semiconductor light-emitting device and method for fabricating the same

ABSTRACT

An n-type buffer layer composed of n-type GaN, an n-type cladding layer composed of n-type AlGaN, an n-type optical confinement layer composed of n-type GaN, a single quantum well active layer composed of undoped GaInN, a p-type optical confinement layer composed of p-type GaN, a p-type cladding layer composed of p-type AlGaN, and a p-type contact layer composed of p-type GaN are formed on a substrate composed of sapphire. A current blocking layer formed in an upper portion of the p-type cladding layer and on both sides of the p-type contact layer to define a ridge portion is composed of a dielectric material obtained by replacing some of nitrogen atoms composing a Group III–V nitride semiconductor with oxygen atoms.

This application is a divisional of Application Ser. No. 10/243,711filed Sep. 16, 2002, now U.S. Pat. No. 6,746,948.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor light-emitting devicecomposed of a Group III–V nitride semiconductor which is capable ofoutputting light ranging in color from blue to ultraviolet and to amethod for fabricating the same.

In recent years, semiconductor light-emitting devices each using a GroupIII–V nitride semiconductor represented by a general formula:B_(x)Al_(y)Ga_(1-x-y-z)In_(z)N (where x, y, and z satisfy 0≦x≦1, 0≦y≦1,0≦z≦1, x+y+z=1), i.e., a light-emitting diode device and a semiconductorlaser device have been developed vigorously as light sources foremitting light ranging in color from blue to ultraviolet.

Referring to the drawings, a conventional semiconductor light-emittingdevice composed of a Group III–V nitride semiconductor will bedescribed.

As shown in FIG. 13, an n-type contact layer 102 composed of n-type GaN,an n-type cladding layer 103 composed of n-type AlGaN, an active layer104 composed of GaInN, a p-type cladding layer 105 composed of p-typeAlGaN, and a p-type contact layer 106 composed of p-type GaN are formedsuccessively on a substrate 101 composed of, e.g., sapphire by epitaxialgrowth.

A current blocking layer 107 composed of a silicon dioxide or a siliconnitride and having an opening 107 a for current confinement is formed onthe p-type contact layer 106. A p-side electrode 108 is formed on theportion of the p-type contact layer 106 exposed through the opening 107a of the current blocking layer 107.

As another method involving the provision of a current confiningstructure, there has been known one which confines a current path byremoving, from a laser device structure, at least the both side portionsof the p-type cladding layer 105 by etching.

In the conventional semiconductor light-emitting device, however, asilicon dioxide or silicon nitride is deposited by chemical vapordeposition or the like to form the current blocking layer 107 on thep-type contact layer 106. Each of the silicon dioxide and siliconnitride has the problems of poor adhesion to a group III–V nitridesemiconductor, a high density of small holes, i.e., a high pinholedensity, and the like.

If the current confining structure is formed by removing the both sideportions of the cladding layer by etching, the electrode should beformed on the top surface of the ridge region (mesa region) formed bythe etching process so that the area of the electrode is reduced. Thiscauses the problem of an increased DC resistance component in a currentpath.

If the conventional semiconductor light-emitting device is asemiconductor laser device, recombined light generated in the activelayer 104 is confined by the current blocking layer 107 composed of adielectric material to the inside of the Group III–V nitridesemiconductor due to a refractive index difference between the activelayer 104 and the semiconductor. Since the refractive index differenceis relatively large and varies discontinuously (stepwise), if therecombined light is to be confined in, e.g., a single lateral mode, thewidth of the opening 107 a (stripe width) of the current blocking layer107 is reduced excessively so that it becomes difficult to optimize thelaser structure. If the stripe width is reduced excessively, the DCresistance component is increased disadvantageously as described above.

In addition, though not shown in the drawings, there are many casesobserved where a conventional semiconductor laser device uses a cavityhaving a ridge structure. In a case with the ridge structure, theefficiency of the light confinement depends on a difference between afirst refractive index inside the ridge region and a second refractiveindex in a region other than the ridge region. In detail, the firstrefractive index means a first effective refractive index determinedaccording to each refractive index and each thickness of thesemiconductor layer composing the active layer and of the semiconductorlayer composing the cladding layer, and the second refractive indexmeans a second effective refractive index determined according to eachrefractive index and each thickness of the semiconductor layer composingthe active layer, the semiconductor layer composing the cladding layerand, for example, a silicon oxide layer or a silicon nitride layercomposing the sides of the ridge structure. In the conventional ridgestructure, the difference between the first refractive index and thesecond refractive index varies discontinuously (stepwise) and the stepdifference is rather large. Because of the large confinement efficiency,the light emitting point of the laser light may displace at a high poweroutput and the configuration of the spot is liable to change when thelight is confined in the cavity under this condition. For this reason,the design for optimizing the laser structure is rather difficult inequipment requiring accurate control of the light emitting point and thespot configuration, such as an optical laser disk device.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to solve theconventional problems and thereby improve the adhesion of a currentblocking layer provided in a semiconductor light-emitting devicecomposed of a Group III–V nitride semiconductor, while reducing apinhole density therein.

To attain the object, a semiconductor light-emitting device according tothe present invention comprises: a semiconductor cladding layer on asubstrate; an active layer formed on the cladding layer; a semiconductorcladding layer formed on the active layer, the semiconductor claddinglayer being composed of a Group III–V nitride; and a current blockinglayer formed in the semiconductor layer to have an opening for exposingthe semiconductor layer therethrough, the current blocking layer beingcomposed of a dielectric material obtained by replacing some of nitrogenatoms composing the semiconductor layer with oxygen atoms.

In the semiconductor light-emitting device according to the presentinvention, the dielectric material obtained by replacing some of thenitrogen atoms composing the semiconductor layer composed of the groupIII–V nitride is used for the current blocking layer so that the currentblocking layer is formed integrally with the semiconductor layer. Thisresolves the problem of the current blocking layer associated with theadhesion thereof to the semiconductor layer and significantly reducesthe pinhole density.

In the semiconductor light-emitting device according to the presentinvention, a composition of oxygen in the current blocking layerpreferably decreases gradually with approach to the active layer.

In the arrangement, the composition of oxygen in the semiconductor layerincreases gradually with distance from the inside of the semiconductorlayer toward the outside thereof so that the refractive index differencebetween the semiconductor layer and the current blocking layer changescontinuously. This allows an increase in the width of an opening in thecurrent blocking layer for effecting single lateral mode confinement ifthe semiconductor light-emitting device is, e.g., a laser device andpermits easy optimization of the laser structure.

In the semiconductor light-emitting device according to the presentinvention, the opening of the current blocking layer preferably has astripe plan configuration.

In that case, the semiconductor layer preferably has a ridge portioncomposing a cavity and the current blocking layer is formed also on sideportions of the semiconductor layer.

In the semiconductor light-emitting device according to the presentinvention, the opening of the current blocking layer preferably has adot plan configuration.

A method for fabricating a semiconductor light-emitting device accordingto the present invention comprises: a first step of forming an activelayer and a cladding layer on a substrate; a second step of forming, onthe active layer, a semiconductor layer composed of a Group III–Vnitride; a third step of selectively forming, on the semiconductorlayer, a mask film for partially masking the semiconductor layer; afourth step of oxidizing the semiconductor layer with the mask filmformed thereon in an oxidizing atmosphere to form, in the semiconductorlayer, a current blocking layer composed of a dielectric materialobtained by replacing some of nitrogen atoms composing the semiconductorlayer with oxygen atoms; and a fifth step of removing the mask film.

In accordance with the method for fabricating a semiconductorlight-emitting device according to the present invention, thesemiconductor layer composed of a Group III–V nitride is formed on theactive layer and the mask film is formed selectively thereon. Thesemiconductor layer is then oxidized by using the formed mask film toform the current blocking layer composed of a dielectric materialobtained by replacing some of the nitrogen atoms composing thesemiconductor layer with oxygen atoms. This prevents a problemassociated with the adhesion between the current blocking layer and thesemiconductor layer. Since some of the nitrogen atoms composing thesemiconductor layer have been replaced with oxygen atoms which arelarger in atomic radius than the nitrogen atoms, the volume of thesemiconductor layer is increased so that a pinhole density in thecurrent blocking layer is reduced significantly.

Preferably, the method for fabricating a semiconductor light-emittingdevice further comprises, between the third and fourth steps: a sixthstep of patterning the semiconductor layer into a ridge configuration byusing the mask.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural cross-sectional view of a semiconductor laserdevice according to a first embodiment of the present invention;

FIGS. 2A to 2D are structural cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlaser device according to the first embodiment;

FIG. 3 is a graph showing the relationship between injected current andlight output in the semiconductor laser device according to the firstembodiment;

FIG. 4 is a graph showing a far field pattern in the semiconductor laserdevice according to the first embodiment;

FIG. 5 is a graph showing the dependence of far field pattern on lightoutput in a direction parallel to the junction surface of thesemiconductor laser device according to the first embodiment;

FIG. 6 is a graph showing the dependence of far field pattern on lightoutput in a direction parallel to the junction surface of a conventionalsemiconductor laser device;

FIG. 7 is a graph showing the results of high-temperature high-humiditylife tests conducted on the semiconductor laser device according to thefirst embodiment and on the conventional semiconductor laser device;

FIG. 8 is a structural cross-sectional view of a semiconductor laserdevice according to a second embodiment of the present invention;

FIGS. 9A to 9E are structural cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlaser device according to the second embodiment;

FIG. 10 is a structural cross-sectional view of a semiconductor laserdevice according to a third embodiment of the present invention;

FIGS. 11A to 11E are structural cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlaser device according to the third embodiment;

FIG. 12 is a perspective view of an array of blue-light-emitting diodedevices according to a fourth embodiment of the present invention; and

FIG. 13 is a structural cross-sectional view of the conventionalsemiconductor laser device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment 1

A first embodiment of the present invention will be described withreference to the drawings.

FIG. 1 shows a cross-sectional structure of a semiconductor laser deviceaccording to the first embodiment.

The semiconductor laser device shown in FIG. 1 has a doubleheterostructure including cladding layers composed of an aluminumgallium nitride (AlGaN) having a relatively large band gap and a quantumwell layer and the like composed of any of a gallium nitride (GaN), agallium indium nitride (GaInN) and a combination thereof each having aband gap smaller than that of the cladding layers, which are sandwichedbetween the cladding layers. A description will be given herein below tothe case where a single quantum well separate confinementheterostructure (SQW-SCH) is used.

As shown in FIG. 1, an n-type buffer layer 11 composed of n-type GaNwith a thickness of about 2 μm, an n-type cladding layer 12 composed ofn-type Al_(0.15)Ga_(0.85)N with a thickness of about 1 μm, an n-typeoptical confinement layer 13 composed of n-type GaN with a thickness ofabout 0.1 μm, a single quantum well active layer 14 composed of undopedGa_(0.8)In_(0.2)N with a thickness of about 5 nm, a p-type opticalconfinement layer 15 composed of p-type GaN with a thickness of about0.1 μm, a p-type cladding layer 16 composed of p-typeAl_(0.15)Ga_(0.85)N with a thickness of about 1 μm, and a p-type contactlayer 17 composed of p-type GaN with a thickness of about 0.5 μm areformed successively on a substrate 10 composed of sapphire(single-crystal Al₂O₃) by crystal growth.

In the structure, the n-type buffer layer 11 alleviates latticemismatching between sapphire composing the substrate 10 and each of thesemiconductor layers 12 to 17 grown epitaxially on the substrate 10,while functioning as a contact layer for an n-side electrode 20.

The n-type cladding layer 12 and the p-type cladding layer 16 confinecarriers injected from the n-side electrode 20 and a p-side electrode19, respectively, while confining recombined light of the carriers. Then-type optical confinement layer 13 and the p-type optical confinementlayer 15 are provided to improve the efficiency with which therecombined light is confined.

The first embodiment is characterized in that a current blocking layer18 composed of a dielectric material obtained by replacing some ofnitrogen atoms composing the p-type cladding layer 16 and the p-typecontact layer 17 with oxygen atoms is formed in an upper portion of thep-type cladding layer 16 in mutually spaced apart relation and on bothsides of the p-type contact layer 17. The current blocking layer 18 isformed such that the p-type cladding layer 16 and the p-type contactlayer 17 form a mesa-like ridge structure having a stripe planconfiguration. The upper part of the ridge portion has a width of about2 μm, while the lower part thereof has a width of about 3 μm.

The p-side electrode 19 composed of a multilayer structure of nickel(Ni), platinum (Pt), and gold (Au) is formed on the p-type contact layer17 to make an ohmic contact therewith. The n-side electrode 20 composedof a multilayer structure of titanium (Ti), platinum (Pt), and gold (Au)is formed on the exposed region of the n-type buffer layer 11 to make anohmic contact with the n-type buffer layer 11.

Thus, the semiconductor laser device according to the first embodimentis a so-called edge-emitting laser having a cavity using the both endsurfaces of the ridge structure as cleaved mirror surfaces. The cavityextends in a direction perpendicular to the cross section of thesemiconductor layer in FIG. 1 (front-to-rear direction).

A description will be given to a method for fabricating thesemiconductor laser device thus constructed with reference to thedrawings.

FIGS. 2A to 2D show cross-sectional structures of the semiconductorlaser device according to the first embodiment in the individual stepsof the fabrication method therefor. In FIGS. 2, the same components asshown in FIG. 1 are designated by the same reference numerals.

First, as shown in FIG. 2A, the n-type buffer layer 11, the n-typecladding layer 12, the n-type optical confinement layer 13, the singlequantum well active layer 14 (hereinafter referred to as the activelayer 14), the p-type optical confinement layer 15, the p-type claddinglayer 16, and the p-type contact layer 17 are grown successively on thesubstrate 10 composed of sapphire by, e.g., metal organic vapor phaseepitaxy (MOVPE), thereby forming epitaxial layers. In the presentembodiment, silicon (Si) is used as an n-type dopant and magnesium isused as a p-type dopant.

Next, as shown in FIG. 2B, a polysilicon film with a thickness of about500 nm is deposited on the p-type contact layer 17 by, e.g., CVD.Subsequently, a stripe resist pattern (not shown) with a width of about3 μm is formed by lithography to cover the region of the p-type contactlayer 17 to be formed with the cavity. By using the formed resistpattern as a mask, dry etching using tetrafluorocarbon (CF₄) as anetching gas is performed with respect to the polysilicon film, therebyforming an anti-oxidant film 90 from the polysilicon film. By using theformed anti-oxidant film 90 as a mask, the epitaxial layer issubsequently exposed to an oxygen atmosphere at a temperature of about900° C. and under a pressure of about 1 atm for about 24 hours, wherebythe upper portion of the p-type cladding layer 16 and the portions ofthe p-type contact layer 17 located on both sides of the anti-oxidantfilm 90 are oxidized. As a result, some of the nitrogen atoms composingthe p-type cladding layer 16 and the p-type contact layer 17 arereplaced with oxygen atoms so that the current blocking layer 18 isformed in the upper portion of the p-type cladding layer 16 and in theportions of the p-type contact layer 17 located on both sides of theanti-oxidant film 90.

The first embodiment is characterized in that the concentrationdistribution of oxygen in the current blocking layer 18 has a profilegradually decreasing with distance from the surface of the currentblocking layer 18 to the inside thereof

Next, as shown in FIG. 2C, the anti-oxidant film 90 is removed byimmersing the epitaxial layer in a buffered hydrofluoric acid which is asolution mixture of a hydrofluoric acid (HF) and an ammonium fluoride(NH₄F). The residual portion of the anti-oxidant film 90 is removed bydry etching using tetrafluorocarbon, while the current blocking layer 18formed is not etched by the buffered hydrofluoric acid. Thereafter, aresist pattern (not shown) for masking the portion of the epitaxiallayer including the cavity formation region is formed by lithography. Byusing the formed resist pattern as a mask, dry etching using chlorine(Cl₂) is performed with respect to the epitaxial layer, thereby exposingthe n-type buffer layer 11.

Next, as shown in FIG. 2D, nickel, platinum, and gold are depositedsuccessively on the p-type contact layer 17 by, e.g., vapor depositionso that the p-side electrode 19 composed of the resulting multilayerstructure is formed selectively. Subsequently, titanium, platinum, andgold are deposited successively on the exposed portion of the n-typebuffer layer 11 so that the n-side electrode 20 composed of theresulting multilayer structure is formed selectively. The order in whichthe p-side electrode 19 and the n-side electrode 20 are formed may alsobe reversed.

Thereafter, the substrate 10 is cleaved such that the cavity has alength of about 500 μm and the emitting edge surface of the cavity isprovided with low reflection coating to have a reflectivity of about10%, while the reflecting edge surface of the cavity is provided withhigh reflection coating to have a reflectivity of about 80%, though theforegoing processes are not depicted.

In accordance with the fabrication method of the first embodiment, thecurrent blocking layer 18 is formed from a dielectric material obtainedby replacing some of the nitrogen atoms composing the p-type claddinglayer 16 and the p-type contact layer 17 with oxygen atoms.Consequently, the adhesion of the current blocking layer 18 to thep-type cladding layer 16 and to the p-type contact layer 17 is improvedsignificantly, which is different from a conventional current blockinglayer composed of a different material such as a silicon dioxide or asilicon nitride. In addition, the density of pinholes occurring in thecurrent blocking layer 18 is also reduced greatly since the silicondioxide (SiO₂) or silicon nitride (Si₃N₄) is not used.

Moreover, the amount of oxygen contained in the current blocking layer18 decreases gradually with approach to the active layer 14. This allowsthe refractive index of the current blocking layer 18 to changecontinuously and increases, e.g., the stripe width (ridge width) forachieving single lateral mode confinement, thereby easily optimizing thelaser structure.

Furthermore, the fabrication method according to the first embodimentallows the current blocking layer 18 to have a light confining functionand a current confining function by self alignment.

The following is the two functions of the current blocking layer 18.

The first one is the current confining function. Because of itsinsulating property, the dielectric layer hardly allows a current toflow therethrough. As a result, the current injected in thesemiconductor laser device flows through the mesa portion (ridgeportion) only into the light-emitting portion of the active layer 14. Asa result, an ineffective current is reduced and the value of a thresholdcurrent for laser oscillation is reduced.

The second one is the light confining function. The p-type claddinglayer 16 and the current blocking layer 18 have a refractive indexdifference therebetween. The refractive index difference confines thelight in the semiconductor laser device to the ridge portion so that thelight is coupled efficiently to induced emission in the active layer 14.

As described above, the conventional current blocking layer composed ofa silicon dioxide or a silicon nitride has a large refractive indexdifference of about 0.5 to 1 between itself and the epitaxial layer,which changes discontinuously. The conventional current blocking layerhas too large a refractive index difference if it is used to control alateral mode in a semiconductor laser device. With slight variations inridge configuration, e.g., a far field pattern causes a higher-orderlateral mode so that it is difficult to use the semiconductor laserdevice as a light source for an optical apparatus.

By contrast, the current blocking layer (dielectric layers) 18 accordingto the first embodiment is composed of a material obtained by thermallydiffusing oxygen from the surface of the epitaxial layer and replacingsome of the nitrogen atoms composing the p-type cladding layer 16 andthe p-type contact layer 17 with oxygen atoms. The current blockinglayer 18 thus formed has a refractive index difference between itselfand the p-tyoe cladding layer 16 which is small and changes gradually sothat the lateral mode in the semiconductor laser device is controlledeasily.

The present inventors measured the composition of oxygen contained inthe current blocking layer 18 by secondary ion mass spectroscopy (SIMS)and verified that the composition of oxygen changes as a quadraticcurve. Specifically, the concentration of oxygen in the near surfaceregion of the current blocking layer 18 is about 1×10²⁰ cm⁻³ and theconcentration of oxygen in the region of the current blocking layer 18at a depth of about 1 μm from the surface thereof is about 1×10¹⁸ cm⁻³.It was verified that, as the composition of oxygen in the currentblocking layer 18 changed as a quadratic curve, the refractive index ofthe current blocking layer 18 also decreased gradually as a quadraticcurve toward the outside thereof, which was about 2.1 in the ridgeportion and about 1.6 in the near surface region of the current blockinglayer 18 with a small difference of about 0.5 therebetween. Thisalleviates confinement of the light to the ridge portion and allowssteady lateral mode oscillation.

FIG. 3 shows the relationship between injected current and light outputin the semiconductor laser device according to the first embodiment. Theoscillation wavelength is 410 nm, while the threshold current andthreshold voltage are 40 mA and 4.2 V. The maximum light output is 60mW. Thus, the current blocking layer 18 composed of a dielectricmaterial obtained by replacing some of the nitrogen atoms composing thep-type cladding layer 16 and the p-type contact layer 17 with oxygenatoms has increased the current confinement efficiency and the lightconfinement efficiency and thereby implemented a reduced thresholdcurrent and a high output operation.

In addition, as shown in FIG. 4, the half width of a far field patternwhen the output is 20 mW is at an angle of 8° with respect to adirection parallel with the junction surface (substrate surface) and atan angel of 25° with respect to a direction perpendicular to thejunction surface.

FIG. 5 shows the dependence of the far field pattern on the light outputin a direction parallel with the junction surface of the semiconductorlaser device having the current blocking layer 18 according to the firstembodiment. FIG. 6 is for comparison and shows the light outputcharacteristic of a far field pattern in a direction parallel with thejunction surface of a semiconductor laser device having a currentblocking layer composed of a silicon nitride according to a conventionalembodiment. The light outputs are 20 mW and 50 mW in each of FIGS. 5 and6.

In the laser device according to the conventional embodiment, arefractive index difference at the interface between the ridge portionand the current blocking layer is about 0.9 and the refractive indexchanges discontinuously at the interface. To obtain fundamental lateralmode oscillation in a direction perpendicular to the junction surface,an effective refractive index difference is reduced by increasing thedistance between the lower part of the ridge portion and the activelayer. In such a structure, however, the injected current expands in thevicinity of the active layer. This not only increases the value of thethreshold current but also renders the confinement of recombined lightunsteady. If the light output value is increased to 50 mW as shown inFIG. 6, the relative intensity has two peaks so that a so-calleddouble-humped far field pattern is observed.

In the laser device according to the first embodiment shown in FIG. 5,by contrast, the relative intensity has one peak even if the outputvalue is 20 mW or 50 mW so that a single-humped far field pattern isobserved.

FIG. 7 shows the result of a high-temperature high-humidity life testconducted on the semiconductor laser device according to the firstembodiment. The life test was conducted under conditions such that theoutput was 30 mW, the temperature was 60° C., and the humidity was 85%.In FIG. 7, the abscissa axis represents an operating time and thecoordinate axis represents an operating current normalized by assumingthat the operating current immediately after the initiation of operationwas 1. The curves A were obtained from the semiconductor laser deviceaccording to the present invention, while the curves B were obtainedfrom the semiconductor laser device according to the conventionalembodiment. As shown in FIG. 7, since the adhesion of the currentblocking layer composed of a silicon nitride to the ridge portion ispoor in the conventional embodiment, peeling off proceeds at the currentblocking layer. This changes the distribution of refractive indices andcauses higher-order lateral mode oscillation. Consequently, the couplingof laser light to induced emission in the active layer is weakened andthe value of the threshold current increases to cause drasticdegradation.

In the semiconductor laser device according to the present invention,the current blocking layer is formed by partly oxidizing the p-typecladding layer 16 and the p-type contact layer 17 composing the ridgeportion instead of using a dielectric material other than that composingthe ridge portion to compose the current blocking layer. Since theadhesion to the ridge portion is remarkably excellent, peeling off isseldom observed so that steady operation is performed over a longperiod.

Although the description has been given to the case where thesemiconductor light-emitting device is the semiconductor laser device,the current blocking layer according to the present invention is alsoapplicable to a light-emitting diode device, similarly to thesemiconductor laser device, in terms of the current confining functionand the light confining function.

Although the edge-emitting laser device has been described as an exampleof the semiconductor laser device, the present invention is alsoapplicable to a surface-emitting laser device.

Embodiment 2

A second embodiment of the present invention will be described hereinbelow with reference to the drawings.

FIG. 8 shows a cross-sectional structure of a semiconductor laser deviceaccording to the second embodiment.

In contrast to the first embodiment which uses insulating sapphire tocompose the substrate 10 on which the epitaxial layer is grown, thesecond embodiment uses, to compose a substrate 30, any of an n-typegallium nitride (GaN), an n-type aluminum gallium nitride (AlGaN), andan n-type silicon carbide (SiC) each having conductivity and close inlattice constant to a Group III nitride semiconductor, as shown in FIG.8.

As shown in FIG. 8, an n-type buffer layer 31 composed of n-type GaNwith a thickness of about 2 μm, an n-type cladding layer 32 composed ofn-type Al_(0.15)Ga_(0.85)N with a thickness of about 1 μm, an n-typeoptical confinement layer 33 composed of n-type GaN with a thickness ofabout 0.1 μm, a single quantum well active layer 34 composed of undopedGa_(0.8)In_(0.2)N with a thickness of about 5 nm, a p-type opticalconfinement layer 35 composed of p-type GaN with a thickness of about0.1 μm, a p-type cladding layer 36 composed of p-typeAl_(0.15)Ga_(0.85)N with a thickness of about 1 μm, and a p-type contactlayer 37 composed of p-type GaN with a thickness of about 0.5 μm areformed successively on the substrate 30 composed of, e.g., the n-typegallium nitride (GaN) by crystal growth.

In the second embodiment also, a current blocking layer 38 composed of adielectric material obtained by replacing some of nitrogen atomscomposing the p-type cladding layer 36 and the p-type contact layer 37with oxygen atoms is formed in an upper portion of the p-type claddinglayer 36 in mutually spaced apart relation and on both sides of thep-typed contact layer 37. The current blocking layer 38 is formed suchthat the p-type cladding layer 36 and the p-type contact layer 37 form aridge-like ridge structure having a stripe plan configuration. The upperpart of the ridge portion has a width of about 2 μm, while the lowerpart thereof has a width of about 3 μm.

A p-side electrode 39 composed of a multilayer structure of nickel,platinum, and gold is formed on the p-type contact layer 37 to make anohmic contact therewith. An n-type side electrode 40 composed of amultilayer structure of titanium, platinum, and gold is formed on thesurface of the substrate 30 opposite to the n-type buffer layer 31.

A description will be given to a method for fabricating thesemiconductor laser device thus constructed with reference to thedrawings.

FIGS. 9A to 9E show cross-sectional structures of the semiconductorlaser device according to the second embodiment in the individual stepsof the fabrication method therefor. In FIGS. 9, the same components asshown in FIG. 8 are designated by the same reference numerals.

First, as shown in FIG. 9A, the n-type buffer layer 31, the n-typecladding layer 32, the n-type optical confinement layer 33, the singlequantum well active layer 34 (hereinafter referred to as the activelayer 34), the p-type optical confinement layer 35, the p-type claddinglayer 36, and the p-type contact layer 37 are grown successively on thesubstrate 30 composed of a gallium nitride by, e.g., MOVPE, therebyforming epitaxial layers. Next, as shown in FIG. 9B, a polysilicon filmwith a thickness of about 500 nm is deposited on the p-type contactlayer 37 by, e.g., CVD. Subsequently, a stripe resist pattern (notshown) with a width of about 3 μm is formed by lithography to cover theregion of the p-type contact layer 37 to be formed with the cavity. Byusing the formed resist pattern as a mask, dry etching usingtetrafluorocarbon as an etching gas is performed with respect to thepolysilicon film, thereby forming an anti-oxidant film 90 from thepolysilicon film.

Next, as shown in FIG. 9C, the epitaxial layer is exposed to an oxygenatmosphere at a temperature of about 900° C. and under a pressure ofabout 1 atm for about 24 hours by using the formed anti-oxidant film 90as a mask, whereby the upper portion of the p-type cladding layer 36 andthe portions of the p-type contact layer 37 located on both sides of theanti-oxidant film 90 are oxidized. As a result, some of the nitrogenatoms composing the p-type cladding layer 36 and the p-type contactlayer 37 are replaced with oxygen atoms so that the current blockinglayer 38 is formed in the upper portion of the p-type cladding layer 36and in the portions of the p-type contact layer 37 located on both sidesof the anti-oxidant film 90. In the second embodiment also, theconcentration distribution of oxygen in the current blocking layer 38has a profile gradually decreasing with distance from the surface of thecurrent blocking layer 38 to the inside thereof.

Next, as shown in FIG. 9D, the anti-oxidant film 90 is removed byimmersing the epitaxial layer in a buffered hydrofluoric acid. Theresidual portion of the anti-oxidant film 90 is removed by dry etchingusing tetrafluorocarbon, while the current blocking layer 38 formed isnot etched by the buffered hydrofluoric acid.

Next, as shown in FIG. 9E, nickel, platinum, and gold are depositedsuccessively by, e.g., vapor deposition in such a manner as to cover thep-type contact layer 37 so that the p-side electrode 39 composed of theresulting multilayer structure is formed selectively. Subsequently,titanium, platinum, and gold are deposited successively on the surfaceof the substrate 30 opposite to the n-type buffer layer 31 so that then-side electrode 40 composed of the resulting multilayer structure isformed selectively. In the present embodiment also, the order in whichthe p-side electrode 39 and the n-side electrode 40 are formed may alsobe reversed.

Thereafter, the substrate 30 is cleaved such that the cavity has alength of about 500 μm and the emitting surface of the cavity isprovided with low reflection coating to have a reflectivity of about10%, while the reflecting surface of the cavity is provided with highreflection coating to have a reflectivity of about 80%, though theforegoing processes are not depicted.

Thus, in the semiconductor laser device according to the secondembodiment and the fabrication method therefor, the current blockinglayer 38 is formed from a dielectric material obtained by replacing someof the nitrogen atoms composing the p-type cladding layer 36 and thep-type contact layer 37 with oxygen atoms. Accordingly, the adhesion ofthe current blocking layer 38 to the p-type cladding layer 36 and to thep-type contact layer 39 is improved significantly

In addition, the amount of oxygen contained in the current blockinglayer 38 decreases gradually with approach to the active layer 34. Thisallows the refractive index of the current blocking layer 38 to changecontinuously and easily optimizes the laser structure.

Moreover, the n-side electrode 40 is provided on the surface of thesubstrate 30 opposite to the n-type buffer layer 31 in opposing relationto the p-side electrode 39. This obviates the necessity for an etchingstep as performed in the first embodiment with respect to the epitaxiallayer to expose the n-type buffer layer 31. As a consequence, theproduction yield is further improved than in the first embodiment.

The second embodiment is not only applicable to an edge-emitting laserdevice but also to a surface-emitting laser device, similarly to thefirst embodiment.

Embodiment 3

A third embodiment of the present invention will be described hereinbelow with reference to the drawings.

FIG. 10 shows a cross-sectional structure of a semiconductor laserdevice according to the third embodiment.

A structure according to the third embodiment is obtainable by furtherimproving the light confining efficiency of the ridge structure of thesemiconductor laser device described in the second embodiment.

As shown in FIG. 10, an n-type buffer layer 51 composed of n-type GaNwith a thickness of about 2 μm, an n-type cladding layer 52 composed ofn-type Al_(0.15)Ga_(0.85)N with a thickness of about 1 μm, an n-typeoptical confinement layer 53 composed of n-type GaN with a thickness ofabout 0.1 μm, a single quantum well active layer 54 composed of undopedGa_(0.8)In_(0.2)N with a thickness of about 5 μnm, a p-type opticalconfinement layer 55 composed of p-type GaN with a thickness of about0.1 μm, a p-type cladding layer 56 composed of p-typeAl_(0.15)Ga_(0.85)N with a thickness of about 1 μm, and a p-type contactlayer 57 composed of p-type GaN with a thickness of about 0.5 μm areformed successively on the substrate 50 composed of, e.g., an n-typegallium nitride (GaN) by crystal growth. A material composing thesubstrate 50 is not limited to a gallium nitride. Instead, an n-typeAlGaN or SiC may also be used to compose the substrate 50.

The third embodiment is characterized in that a region extending fromthe p-type contact layer 57 to the upper portion of the n-type claddinglayer 52 has its both side portions etched to form a mesa portion (ridgeportion) having a width of about 2 μm. A current blocking layer 58composed of a dielectric material obtained by replacing some of nitrogenatoms composing the region extending from the p-type contact layer 57 tothe n-type cladding layer 52 with oxygen atoms is formed on both sidesof the ridge portion.

A p-side electrode 59 composed of a multilayer structure of nickel,platinum, and gold is formed on the p-type contact layer 57 to make anohmic contact therewith. An n-side electrode 60 composed of a multilayerstructure of titanium, platinum, and gold is formed on the surface ofthe substrate 50 opposite to the n-type buffer layer 51.

Although the p-side electrode 59 is formed only on the upper surface ofthe p-type contact layer 57, the p-side electrode 59 may also beprovided to cover the upper portion of the current blocking layer 58.

A description will be given to a method for fabricating thesemiconductor laser device thus constructed with reference to thedrawings.

FIGS. 11A to 11E show cross-sectional structures of the semiconductorlaser device according to the third embodiment in the individual stepsof the fabrication method therefor. In FIGS. 11, the same components asshown in FIG. 10 are designated by the same reference numerals.

First, as shown in FIG. 11A, the n-type buffer layer 51, the n-typecladding layer 52, the n-type optical confinement layer 53, the singlequantum well active layer 54 (hereinafter referred to as the activelayer 54), the p-type optical confinement layer 55, the p-type claddinglayer 56, and a p-type lower contact layer 57 a are grown successivelyon the substrate 50 composed of a gallium nitride by, e.g., MOVPE,thereby forming an epitaxial layer. Since the p-type lower contact layer57 a will be regrown in the subsequent step, the thickness thereof islimited to about 0.3 μm.

Next, as shown in FIG. 11B, a polysilicon film with a thickness of about500 nm is deposited on the p-type lower contact layer 57 a by, e.g.,CVD. Subsequently, a stripe resist pattern (not shown) with a width ofabout 3 μm is formed by lithography to cover the region of the p-typelower contact layer 57 a to be formed with the cavity. By using theformed resist pattern as a mask, dry etching using tetrafluorocarbon asan etching gas is performed with respect to the polysilicon film,thereby forming an anti-oxidant film 90 from the polysilicon film.Subsequently, dry etching using chlorine (Cl₂) is performed with respectto the epitaxial layer by using the resist pattern and the anti-oxidantfilm 90 as a mask till the n-type cladding layer 52 is exposed, therebyforming the ridge portion.

Next, as shown in FIG. 11C, the epitaxial layer is exposed to an oxygenatmosphere at a temperature of about 900° C. and under a pressure ofabout 1 atm for about 24 hours by using the formed anti-oxidant film 90as a mask, whereby the upper portion of the n-type cladding layer 52located on both sides of the ridge portion and the respective portionsof the n-type optical confinement layer 53, the active layer 54, thep-type optical confinement layer 55, the p-type cladding layer 56, andthe p-type lower contact layer 57 a located on both sides of the ridgeportion are oxidized. As a result, some of the nitrogen atoms composingthe region extending from the p-type lower contact layer 57 a to theupper portions of the n-type cladding layer 52 are replaced with oxygenatoms so that the current blocking layer 58 is formed on both dies ofthe region extending from the p-type lower contact layer 57 a to theupper portion of the n-type cladding layer 52.

Next, as shown in FIG. 11D, the anti-oxidant film 90 is removed byimmersing the epitaxial layer in a buffered hydrofluoric acid. Theresidual portion of the anti-oxidant film 90 is removed by dry etchingusing tetrafluorocarbon, while the current blocking layer 58 formed isnot etched by the buffered hydrofluoric acid. Subsequently, the p-typelower contact layer 57 a is regrown by MOVPE to form the p-type contactlayer 57 with a thickness of about 0.5 μm. The p-type contact layer 57is grown to cover the upper end surfaces of the current blocking layer58.

Next, as shown in FIG. 11E, nickel, platinum, and gold are depositedsuccessively by, e.g., vapor deposition in such a manner as to cover thep-type contact layer 57 so that the p-side electrode 59 composed of theresulting multilayer structure is formed selectively. Subsequently,titanium, platinum, and gold are deposited successively on the surfaceof the substrate 50 opposite to the n-type buffer layer 51 so that then-side electrode 60 composed of the resulting multilayer structure isformed selectively. In the present embodiment also, the order in whichthe p-side electrode 59 and the n-side electrode 60 are formed may alsobe reversed.

Thereafter, the substrate 50 is cleaved such that the cavity has alength of about 500 μm and the emitting surface of the cavity isprovided with low reflection coating to have a reflectivity of about10%, while the reflecting surface of the cavity is provided with highreflection coating to have a reflectivity of about 80%, though theforegoing processes are not depicted.

Thus, in the semiconductor laser device according to the thirdembodiment and the fabrication method therefor, the current blockinglayer 58 is formed from a dielectric material obtained by replacing someof the nitrogen atoms composing the region extending from the n-typecladding layer 52 to the p-type contact layer 57 with oxygen atoms.Accordingly, the adhesion of the current blocking layer 58 to the ridgeportion is improved significantly.

In addition, the amount of oxygen contained in the current blockinglayer 58 decreases gradually with approach to the active layer 54. Thisallows the refractive index of the current blocking layer 58 to changecontinuously and easily optimizes the laser structure.

Moreover, since the n-side electrode 60 is provided on the surface ofthe substrate 50 opposite to the n-type buffer layer 51 in opposingrelation to the p-side electrode 59, a DC resistance component isreduced.

Further, since the p-side electrode 59 is provided over the currentblocking layer 58, an alignment step in forming the electrode is no morenecessary.

The third embodiment is not only applicable to an edge-emitting laserdevice but also to a surface-emitting laser device, similarly to thefirst and second embodiments-.

Embodiment 4

A fourth embodiment of the present invention will be described hereinbelow with reference to the drawings.

FIG. 12 diagrammatically shows an array of blue-light-emitting diodedevices according to the fourth embodiment.

As shown in FIG. 12, an epitaxial layer 71 composed of a doubleheterostructure equal to that used in the second embodiment is formed ona substrate 70 composed of any of a gallium nitride (GaN), an aluminumgallium nitride (AlGaN), and a silicon carbide (SiN) each having n-typeconductivity.

A current blocking layer 72 is formed on the upper surface of theepitaxial layer 71 in such a manner that a p-type contact layer 71 a isexposed through four circular openings 72 a arranged as a matrix.

The fourth embodiment is characterized in that the current blockinglayer 72 is formed from a dielectric material obtained by replacing someof nitrogen atoms composing at least the p-type contact layer 71 a withoxygen atoms.

A transparent electrode 73 composed of, e.g., an ITO (Indium Tin Oxide)is formed on the current blocking layer 72 to make an ohmic contact withthe p-type contact layer 71 a exposed through the openings 72 a andtransmit recombined light such that output light is retrieved from abovethe epitaxial layer. An n-side electrode 74 composed of a multilayerstructure of titanium, platinum, and gold is formed on the surface ofthe substrate 70 opposite to the epitaxial layer 71.

It is to be noted that the current blocking layer 72 provides electricalisolation between the individual diode devices.

According to the fourth embodiment, the light can be confined opticallyto the region under the opening of the current cladding layer 72 in theepitaxial layer 71 since the current blocking layer 72 is lower inrefractive index than the epitaxial layer 71.

A multi-layer dielectric film which transmits emission wavelengths mayalso be provided on the transparent electrode 73. In this case, aso-called surface-emitting laser structure may also be adopted in whicha periodic structure having reflectivity against emission wavelengths isprovided under the double heterostructure of the epitaxial layer 71 tocause laser oscillation.

Since the fourth embodiment uses the current blocking layer 72 formed byreplacing some of nitrogen atoms composing a Group III–V nitridesemiconductor with oxygen atoms, similarly to the first embodiment, thereliability of an isolation region is increased significantly comparedwith the case where the conventional current blocking layer composed ofa silicon nitride is used.

Hence, the array of the blue-light-emitting diode devices according tothe fourth embodiment offers high reliability even if it is used intough outdoor environments.

In each of the first to fourth embodiment, the current blocking layerformed by replacing some of nitrogen atoms composing a Group III–Vnitride semiconductor with oxygen atoms can achieve similar effects ifit is applied to a white light source obtained by exciting a fluorescentmaterial, an optical integrated device, or the like.

1. A semiconductor light-emitting device comprising: an active layerformed on a substrate; a semiconductor layer formed on the active layer,the semiconductor layer being composed of a Group III–V nitridecontaining Ga; and an insulative current blocking layer formed in thesemiconductor layer to have an opening for exposing the semiconductorlayer therethrough, the insulative current blocking layer comprising adielectric material obtained by replacing some of nitrogen atomscomposing the semiconductor layer with oxygen atoms by heat treatment,at least some of the nitrogen atoms bonding with Ga, wherein acomposition of oxygen in the insulative current blocking layer decreasesgradually with approach to the active layer, and the insulative currentblocking layer is separate from the active layer.
 2. The semiconductorlight-emitting device of claim 1, wherein the opening of the insulativecurrent blocking layer has a stripe plan configuration.
 3. Thesemiconductor light-emitting device of claim 1, wherein the opening ofthe insulative current blocking layer has a circular configuration.